Formation of chromium-containing coatings on steel strip



United States Patent "ice 3,340,054 FORMATION OF CIROMIUM-CONTAINING COATINGS 0N STEEL STRIP George W. Ward, Nazareth, and Richard M. Wilhson,

Bethlehem, Pa., assignors to Bethlehem Steel Corporation, a corporation of Delaware No-Drawing. Continuation of applicat on Ser. No. 500,325, Oct. 21, 1965. This application Sept. 20, 1966, Ser. No. 580,607

6 Claims. (Cl. 75208) ABSTRACT OF THE DISCLOSURE In a process of forming a chromium-containingcoating on steel strip, a chromium-containing powder is compasted on to the strip, and the strip and compacted powder are sintered in a protective hydrogen atmosphere to which chlorine gas is supplied in an amount not less than 0.10% by volume of said atmosphere.

This application is a continuation of Ser. No. 500,325 filed Oct. 21, 1965 and now abandoned, which, in turn, is a continuation-in-part of application Ser. No. 371,127, filed May 28, 1964, and now abandoned.

This invention relates to the formation of a chromiumcontaining coating on steel strip, and more particularly to the formation of an iron-chromium alloy coating.

The principal object of this invention is to produce an adherent, protective iron-chromium alloy layer on the surface of steel articles such as strip, sheets, plates, rods, bars and wire.

Another object is to produce a diffused chromium alloy coating on a steel base, wherein diffusion takes place in a protective atmosphere.

A further object is to produce a diifused chromium alloy coating on a steel base, wherein diffusion takes place in a protective atmosphere, which is augmented with controlled amounts of chlorine gas.

In the production of a chromium alloy coating on a steel strip, or similar steel article, by compacting chromium-containing powder on the steel article, and sintering tofdiifuse the powder with the base metal, sintering should take place, preferably, in a substantially 100% pure hydrogen atmosphere. It is possible to maintain such an "atmosphere under ideal operating conditions, but in large scale operations, where, for example, a 36 inch wide strip, of a coil length weighing four tons .or more, is chromized by a compacted-sintered powder technique, the sintering atmosphere will frequently be contaminated with impurities which affect the sintering process adversely. Impurities may be introduced into the sintering atmosphere of a sintering, or annealing, furnace through leaks in the system, or from refractories used in the furnace base.'In a large scale coating operation, it may be difiicult to obtain sufficient hydrogen of the required purity. Contaminants most likely to be found in the sintering atmosphere are oxygen, carbon and nitrogen. Chromium powder, or chromium-containing powder, which has been compacted on the metal base prior to sintering, has a strong aflinity for any of the elements mentioned above as contaminants.

The contaminant most likely to be found in the sintering atmosphere is, of course, oxygen. Oxygen is particularly-troublesorne in a sintering operation of the sort utilized in powder coating of strip, because it has been found to interfere with the diffusion of the chromium into the iron of the base steel, and of the iron into the compacted chromium-containing powder.

In a sintering furnace, containing a large coil of steel strip,both'- sides of which are covered with a compacted chromium-containing powder, the strong aflinity of chro- 3,340,054 Patented Sept. 5, 1967 mium for oxygen results in a preferential reaction between the oxygen and the chromium. Even small amounts of oxygen in the furnace atmosphere will be found to react almost entirely with the compacted chromium. In reacting, the oxygen forms an oxide film on the exposed chromium surfaces, and efiectively blocks the chromium particles so reacted from diffusing into the iron of the strip base. Similarly, the iron-of the base metal is retarded in reacting with the compacted powder. Additionally, when a mixture of chromium and iron powder, or a chromium-iron alloy powder is used as the coating medium, oxidation of the chromium in the powder prevents ready diffusion of the chromium and iron particles of the compacted material itself.

An accumulation of oxide film on the chromium particles of the compacted coating, due to impurities in the sintering atmosphere, can be so severe that no continuous chromium alloy coating can be formed in any reasonable sintering time cycle, even though the sintering temperature is maintained far above that ordinarily required for chromium-iron diffusion.

We have found that in the manufacture of chromium alloy coatings on steel strip by the method of compacting chromium-containing powder on the strip and sintering the article, excellent coatings of stainless steel quality and uniform thickness can be obtained, by the controlled introduction of chlorine gas into the annealing furnace during the sintering operation. The chlorine may be introduced into the furnace independently of, or in conjunction with, the hydrogen gas, which gas is used to maintain a reducing atmosphere during the heating-up, sintering or soaking, and cooling cycles. a

To illustrate the manner by which chlorine can be employed as an energizer, in the formation of a chromium alloy coating on a linear, or strip-type, steelbase, a 9790 pound coil of 18 gage low carbon sheet steel (0.003% carbon) was filmed with a liquid, in this case tridecyl alcohol, and the filmed strip was passed through a fluidized bed of a chromium-iron alloy powder. The alloy powder used was that known as Simplex ferrochi'ome, having an analysis of 71.3% chromium, 0.35% manganese, 1.42% silicon, 0.01% carbon and the balance iron. The powder had a particle size represented by that which passes a 200 mesh screen (U.S'. Standard Sieve Series). The strip emerged from the fluidized bed compartment with a uniform coating of the alloy powder on both sides of the strip, the approximate mean thickness of the coating being 0.001 inch. The strip was next passed through the rolls of a temper mill, having 28 inch diameter rolls, to compact the powder on the strip. In this mill, the powder is pressed into a porous, semi-adherent metal shell, in which the particles of powder, immediately adjacent the strip base, are firmly forced into mechanical adherence with the base.

Due to the porous nature of the compacted powder, oxygen, or oxide, impurities in the ambient air may react with exposed surfaces of the chromium in said powder, forming a chromium oxide film on the particles.

After compacting, the coil was open-wound with a 0.024- inch zig-zag spacing wire placed about 2 inches below the top edge of the coil. Y

Thus prepared, the coil was ready for the sintering step, and to this end was placed on edge on the pedestal, or base, of a Lee-Wilson type annealing furnace. In this type of furnace, a stainless steel inner cover fits over the base and coil. The inner cover, which contains the atmosphere surrounding the treated coil, has a gas volume capacity of approximately 585 cubic feet. Gas lines, for the introduction of hydrogen and chlorine, exit through the base which holds the coil. The inner cover is sealed at the base with a liquid metal (Woods metal). A steel, refractory-lined furnace'wall encloses the inner cover, and completes the furnace structure. Gas-fired radiant tubes, in the furnace wall, supplying the heat to maintain the temperature within the inner cover which surrounds the charge. The temperature is checked by thermocouples inserted in the base, and extending into the coil.

The inner cover atmosphere was purged with a 4% hydrogen96% nitrogen dry gas at a rate of 900 cu. ft. per hour, :until the concentration of oxygen was reduced to less than 1.0%, and the dew point of the exhaust gas had decreased to 40 F. At that point, the gas was turned OE, and substantially 100% hydrogen introduced at the rate of 900 cu. ft. per hour. After the concentration of hydrogen reached 75%, the furnace was placed on the base and the heating-up cycle begun.

, During the early stages of the heating-up period, tests were made periodically to determine the dew point in the treatment atmosphere within the inner cover. After 2 /2 hours the exit dew point had increased to 20 F., then gradually declined to 45 F. at 800 R, which temperature was maintained for about 5 hours. The temperature was then gradually increased and chlorine gas was introducedinto the sintering atmosphere along with the hydrogen. The chlorine feed rate was 9 cu. ft. per hour. As the pressure within the inner cover was maintained at substantially atmospheric, exhaust gas left the working area surrounding the treated coil at about the same rate as that of the incoming hydrogen and chlorine.

When the inner cover reached 1700 F., the temperature was held there for for a period of 24 hours. This 24 hour period represents the soaking period, during which diffusion takes place between the chromium in the powder and the iron in the steel strip base. Diffusion also takes place between the chromium in the powder and any iron or nickel contained therein. Chlorine and hydrogen continued to flow into the treatment area, at the aforesaid rates, during the first 12 hours of the soaking period, after which the flow of chlorine was discontinued. The flow of hydrogen was continued during the latter half of the soaking period, and during the major portion of the cooling cycle as well. The furnace heat was turned off at the end of the 24 hour soaking period, and the entire furnace unit permitted to cool. At the end of about 3 hours, the coil had cooled to about 1300 F., at which point the furnace shell was removed and replaced with a cooling cover. When the coil had cooled to 175 F., about 8% hours later, introduction of hydrogen was discontinued, and gas containing 4% hydrogen-96% nitrogen introduced at a rate of 900 cu. ft. per hour. Cooling of the coil continued to 135 F., at which point the charge was uncapped.

As a result of the treatment just described, a chromized coating, averaging 0.002 inch in thickness was produced, which withstood immersion in boiling 20 volume percent nitric acid without signs of failure. This is in contrast with a powder'cornpacted coil of strip treated in the same furnace, and under similar conditions, but in the absence of chlorine. In this latter treatment, the sintered coating was spotty,'having large areas'void of any coating, and other areas in which the coating was porous.

The introduction of chlorine gas into the hydrogen treating atmosphere is believed to reduce the oxides on the powder particles before diffusion begins, and, in addition, to prevent further oxidation during the heatingup and sintering steps.

In another example, a 5000 pound coil of 37 /2 inch width, 20 gage strip was filmed with alcohol, as in the previous example, and coated with Simplex ferrochrornium powder. Another 5000 pound coil, of the same size and grade as the first coil, was similarly filmed with alcohol and coated with regular low carbon ferrochromium powder (0.08% carbon, 70.9% chromium, 0.43% silicon and 0.50% manganese). The two coils were then joined into a 10,000 pound coil, open-wound with a 0.024 inch wire spacer, placed on the base of the annealing furnace,

and covered with the inner cover. The cover was sealed and the furnace placed over it. Before firing, the inner cover atmosphere was purged with 4% hydrogen-96% nitrogen gas. During the heating-up period, the dew point of the exit gas was measured every hour up to a temperature of 800 F. To insure a dry atmosphere, the coil temperature was held at 800 F. for five hours. At the end of this five hour period, chlorine gas was introduced into the atmosphere surrounding the charge at the rate of 4.5 cu. ft. per hour, equivalent to 0.5% of the total volume of the sintering atmosphere. The temperature of the coil was raised gradually to 1700" F., and the flow of chlorine gas into the sintering atmosphere continued at the rate of 4.5 cu. ft. per hour. During the entire heating-up period, hydrogen was introduced into the furnace atmosphere at the rate of 900 cu. ft. per hour. The total soaking time for the sintering operation at 1700 F. was 24 hours, during which the hydrogen flow was maintained. The flow of chlorine was cut olf midway of the sintering operation, or after 12 hours at 1700 F. As in the previous example, at the end of the 24 hour soaking period, the heat supply was turned off in the furnace, and the charge allowed to cool. Hydrogen flow was maintained until the charge reached a temperature of 290 F., at which point 4% hydrogen96% nitrogen gas was introduced for the remainder of the cooling period in place of hydrogen.

Satisfactory chromized coated strip, resistant to boiling 20 volume percent nitric acid, was obtained from both 5000 pound sections of the composite coil.

.To obtain the most effective results with the use of chlorine in this invention, the atmosphere surounding the coil should be substantially free of moisture before the sintering temperature is reached, as any water vapor in the treating atmosphere increases the possibility of oxidation of chrominum particles. As shown in the examples, it is preferable to lower the dew point of the atmosphere to about -40 F. before sintering.

With the introduction of chlorine to the treatment atmosphere, higher levels of water vapor, nitrogen and carbon dioxide, as well as oxygen, can be tolerated. During the heating-up cycle, impurities will often be found to be present in the treating atmosphere to a considerable degree, and at this point may form inhibiting chromium or iron compounds, which later prevent diffusion. Chlorine is found to be most effective during the heating-up period and the early stages of sintering, as it is during these heating stages that the chlorine reacts with the oxide film, or other compound film, on the compacted metal particles, and thus enables elemental chromium to diffuse rapidly into the strip.

In the two examples, the amount of chlorine used represented, in one case 1.0% of the hydrogen treating atmosphere, while in the other, chlorine was introduced in an amount equivalent to 0.5% of the hydrogen atmosphere. Lesser amounts of chlorine may be used, down to about 0.10%, as obviously the amount of chlorine will depend on the amount of contaminants present.

Two considerations indicate the upper limit of chlorine. First, in Bureau of Mines Bulletin No. 503, entitled Limits of Flammability of Gases and Vapors it is shown, at page 23 that chlorine and hydrogen are known to produce an explosive effect when the chlorine content is above 11%.

Secondly, it has been discovered that, in certain instances, an erosive condition may be created in the presence of an excessive amount of chlorine. The introduction of chlorine in large amounts apparently creates a physical disturbance on sections of the open-wound coil, and consequently produces an uneven distribution of the alloy coating layer. While this uneveness of the coating has no effect on the surface appearance, nor on the resistance of the coating to boiling nitric acid in a static test, it may produce localized areas where the coating is quite thin. Thin spots in the coating would limit the amount of deformation or surface finishing, which could be performed on the coated article, as the thin areas would tend to split or rupture more readily than the remainder of the coating.

To avoid erosion of the coating, the chlorine introduced during sintering may be limited to a rate of 0.5% by volume of the total sintering atmosphere. During the heating-up period, prior to sintering, chlorine may be introduced at a rate of 1.0% of the total volume of treating atmosphere, or higher if desired.

As chlorine is highly toxic, the usual industrial precautions should be observed for the handling of this gas, to prevent leakage of any chlorine into areas occupied by operating personnel.

The introduction of chlorine gas to the treating atmosphere should be discontinued before the charge is cooled. This may be done at the termination of sintering, or preferably at some point during sintering, as, for example, midway of the sintering cycle. Chlorine should be exhausted from the atmosphere surrounding the charge before the cooling cycle begins, to prevent formation of chlorine compounds which might condense and have a deleterious effect on the surface of the alloy coating.

In the practice of our invention, the type of metal powder which may be applied to the base metal includes chromium, iron-chromium alloy, a mixture of iron and chromium, iron-chromium alloy plus nickel, a mixture of chromium and nickel, iron-chromium nickel alloy, and a mixture of iron, chromium and nickel powders. In the case of the alloy powders, iron, chromium or nickel may be added if desired. Any combination or mixture of these powders is also suitable as long as chromium is present in the mix in an amount suflicient to produce a chromiumiron alloy in the coating. For the production of a stainless alloy coating, the chromium content of the powder should be at least 20 percent by weight. Incidental amounts of inert material or other substance not adversely affecting the process can be tolerated.

A truly continuous, pore-free, stainless-type coating will resist boiling 20 volume percent aqueous solution of nitric acid (based on 100% I-INO In order to obtain corrosion resistance of this magnitude, the coating must be relatively free of porosity.

Unless the strip and applied powder have a carbon content below a certain predetermined maximum, the composite article is decarburized before the sintering operation. The base strip should have not more than 0.01% effective carbon by weight at the time of sintering. The carbon content of the applied powder should not be more than 0.25% by weight in order to produce a desired coating.

The term effective carbon, as used here, refers to that carbon in either the base metal or the coating, which is available to produce deleterious chromium carbides. These carbides embrittle the coating, and thus limit the formability of the coated product. In addition a coating containing carbides has lower corrosion resistance than a coating free of carbide.

If it is desired to incorporate the strength characteristics of a carbon steel in the coated strip, it is possible to use a strip analyzing more than 0.01% carbon, if a sequestering agent such as titanium is present in the strip. If titanium is present in suflicient quantity to combine with substantially all of the carbon in the strip, it will prevent the carbon in the strip from diffusing to the coating during sintering and forming undesirable chromium carbide in the coating. Other carbide formers which may be used to tie up the carbon are chromium and columbium. When carbide formers are used in the strip to tie up the carbon in this manner, it is still essential that the unbound, eifective carbon in solution in the strip, that which is free to react with the chromium in the compacted powder, be held to a quantity not in excess of 0.01%.

The total efiective carbon, in both the metal strip and the metal powder, should be sufliciently low during 7 sintering, so that the resultant alloy coating will meet the test of resisting attack of a boiling 20 volume percent solution of nitric acid.

It has been shown by analysis that the coating produced is made up entirely of iron-chromium alloy, or, when nickel is used also as a starting material, of an ironchromium-nickel alloy. It is believed that the alloy is formed by a metal to metal diffusion or iron and chro mium, or of iron, chromium and nickel. To obtain the proper degree of diffusion, so that an alloy coating of commercial acceptance is produced, the temperature during sintering should preferably range between approximately 1550" F. and 1900 F. At 1550 F. a minimum time cycle of 12 hours should be used, although considerably longer times may be desirable, depending on the amount of alloying desired, i.e. the amount of chromium in the alloy at the surface of the coating. There is no upper limit for sintering temperature, other than that which may be dictated by practical considerations. At temperatures above 1550 F., the minimum time required will be lowered in an inverse manner.

The proper use of chlorine in the sintering atmosphere not only permits the chlorine to act as an energizer in enabling diffusion to proceed rapidly, even when contaminants in the form of oxygen or oxides are present in the sintering atmosphere, but it has been found that with the energizer, a higher percentage. of carbon can be tolerated in the applied powder.

Another effect, attributable to the use of chlorine, is the vastly increased ductility of the coated product. An alloy coated product made by our method can be reduced in cross section by as much as This effect is of considerable importance in the manufacture of thin gage coated products, for the thickness of the base steel strip is limited during sintering to a gage which will prevent collapse of the coil at the sintering temperature, or a minimum strip thickness of about 0.018 inch.

As has been described above, the stainless coating of this invention is produced through inter-diffusion of chromium into the steel base and of iron from the base into the compacted powder during sintering. In the appended claims, the stainless coating relates to a coating formed by such diffusion during sintering.

Also, in the appended claims, the designation steel strip is meant to include steel sheet or plate as well, and all percentages refer to weight percent except for those relating to chlorine. Chlorine percentages represent volume. i

We claim:

1. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, introducing the strip and compacted metal powder thereon into a sintering furnace, establishing a protective atmosphere therein, introducing chlorine gas into said furnace to bring the chlorine content of said protective atmosphere to not less than 0.10 percent by volume, and sintering said strip and compacted metal powder in said atmosphere for a time and at a temperature sufficient to cause dilfusion between the strip and the powder and to form an adherent stainless steel coating, and maintaining the effective carbon content of said strip at not more than 0.01% during said sintering.

2. The method according to claim 1 in which the sintered strip is cooled in a hydrogen atmosphere free of chlorine.

3. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25 carbon, compacting the powder on the strip, heating up and sintering the strip and compacted powder in a sintering zone in a hydrogen atmosphere, introducing chlorine gas into said atmosphere during the heating up and at least the early stages of the sintering operation in an amount representing no less than 0.10% by volume of said hydrogen atmosphere, removing said chlorine from said sintering zone before sintering is completed and maintaining the effective carbon content of the strip at not over 0.01% during sintermg.

4. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, heating the strip and compacted powder to a temperature not less than 1550" F. and sintering the strip and compacted powder in a continuously flowing hydrogen atmosphere, introducing chlorine gas into said atmosphere in which the chlorine represents not less than 0.10% by volume of said atmosphere during the heating and at least the early stages of sintering, maintaining the effective carbon content of said strip at not more than 0.01% during said sintering, and cooling the strip in a continuously flowing hydrogen atmosphere free of chlorine to a temperature at least as low as 500 F.

5. The method according to claim 3 in which the moisture content of the sintering atmosphere is maintained at a dew point not greater than 40 F. during sintering.

6. The method of forming a coating on steel strip which comprises applying to the surface of the strip a metal powder containing not less than 20% chromium and not more than 0.25% carbon, compacting the powder on the strip, heating up and sintering the strip and compacted powder in a sintering zone in a hydrogen atmosphere, introducing chlorine gas into said atmosphere during the heating up stage in an amount representing not less than 0.10% by volume of said hydrogen atmosphere, and continuing introduction of chlorine gas into said atmosphere during the sintering stage in'an amount representing not more than about 0.5% by volume of said hydrogen atmosphere, removing said chlorine from said sintering zone before sintering is completed, and maintaining the effective carbon content of the strip at not over 0.01% during sintering.

References Cited UNITED STATES PATENTS 1,365,499 1/1921 Kelley 29'-196 .6 1,853,369 4/1932 Marshall 29196.6 2,622,043 12/1952 Roush 11710 '7.2 2,791,517 5/1957 Becker et al. 117107.2 2,851,375 9/1958 Samuel 11750 3,093,556 6/1963 Machu et al 29-196.1 X 3,222,212 12/1965 Samuel et al. 117107.2

L. DEWAYNE RUTLEDGE, Primary Examiner.

CARL D. QUARPORTH, Examiner.

M. J. SCOLNICK, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,340 ,054 September 5 1967 George W. Ward et a1 that error appears in the above numbered pat- It is hereby certified 6. that the said Letters Patent should read as ent requiring correction an corrected below.

Column 3 line 2 for "supplying" read supply columr 6, line 7, for "diffusion or iron" read diffusion of iron Signed and sealed this 1st day of October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. THE METHOD OF FORMING A COATING ON STEEL STRIP WHICH COMPRISES APPLYING TO THE SURFACE OF THE STRIP A METAL POWDER CONTAINING NOT LESS THAN 20% CHROMIUM AND NOT MORE THAN 0.25% CARBON, COMPACTING THE POWDER ON THE STRIP, INTRODUCING THE STRIP AND COMPACTED METAL POWDER THEREON INTO A SINTERING FURNACE, ESTABLISHING A PROTECTIVE ATMOSPHERE THEREIN, INTRODUCING CHLORINE GAS INTO SAID FURNACE TO BRING THE CHLORINE CONTENT OF SAID PROTECTIVE ATMOSPHERE TO NOT LESS THAN 0.10 PERCENT BY VOLUME, AND SINTERING SAID STRIP AND COMPACTED METAL POWDER IN SAID ATMOSPHERE FOR A TIME AND AT A TEMPERATURE SUFFICIENT TO CAUSE DIFFUSION BETWEEN THE STRIP AND THE POWDER AND TO FORM AN ADHERENT STAINLESS STEEL COATING, AND MAINTAINING THE EFFECTIVE CARBON CONTENT OF SAID STRIP AT NOT MORE THAN 0.01% DURING SAID SINTERING. 